MAINT: add free threading support to complex.extension (#655)

Add support for the experimental free-threading Python version in the
complex.extension module.

closes #654

Author: jjhelmus

tests/packages/complex/extension.c

@@ -22,5 +22,12 @@ static struct PyModuleDef module = {
 };
 
 PyMODINIT_FUNC PyInit_extension(void) {
-    return PyModule_Create(&module);
+    PyObject *m = PyModule_Create(&module);
+    if (m == NULL) {
+        return NULL;
+    }
+#ifdef Py_GIL_DISABLED
+    PyUnstable_Module_SetGIL(m, Py_MOD_GIL_NOT_USED);
+#endif
+    return m;
 }